1. Field of the Invention
This invention relates to a lead frame, and particularly, to a multi-layer lead frame having at least one metal plate or plane, hereinafter referred to as "metal plane", used for a semiconductor device.
2. Description of the Related Art
A conventionally known multi-layer lead frame for a semiconductor device comprises at least one metal plane connected via an insulative layer to the bottom surfaces of inner leads, and the use of such a multi-layer lead frame enables the mounting of a relatively power-consumable semiconductor chip on the metal plane, due to a heat radiation of the metal plane. Also, the metal plane can be used as a ground layer or a power supply layer, to thereby improve the electrical characteristics of the semiconductor device.
FIG. 3 shows a conventionally known multi-layer lead frame which includes a ground metal plane 10, a power supply metal plane 12 and a plurality of inner leads 14, which are affixed or laminated to each other, in turn, by electrically insulation films 16a and 16b respectively, made of, for example, polyimide.
The ground plane 10 is also used as a stage for mounting thereon a semiconductor chip (not shown). The power supply plane 12 has a frame-like shape for embracing the semiconductor chip mounting stage, and the inner leads 14 are arranged around the power supply plane 12. Inner tips of the inner leads 14 are arranged so as to be retracted from the front edges of the power supply plane 12, to thus ensure wire-bonding areas for a wire-bonding connection between the semiconductor chip and the exposed surface of the power supply plane 12.
Also, the ground plane 10 and the power supply plane 12 have respective projections 31 (FIG. 5A) protrudind from the outer edges thereof, for an electrical connection thereof to the ground leads and the power supply leads, respectively, among the plurality of inner leads 14.
In the above-mentioned conventional multi-layer lead frame, the dimensions of the outer and inner edges of the insulation film 16a and the inner edge of the insulation film 16b are the same as those of the outer and inner edges of the ground plane 10 and the inner edge of the power supply plane 12, respectively. Consequently, when the ground plane 10 and the power supply plane 12 are abutted against each other, an insulation film having exactly the same dimensions as the power supply plane 12 should be used.
The inner leads 14 and the metal planes used for a multi-layer lead frame can be made not only by an etching process but also by a punching process, which is more suitable for mass-production. Nevertheless, when producing these elements by the punching process, burrs or flashing may be generated, which may cause an electrical short circuit between the inner leads 14 and the metal planes, i.e., between the metal layers.
FIGS. 2A and 2B show burrs or flashing of the inner lead 14. First, when stamping to make the inner leads, extending in the vertical direction may be generated burrs or flashing 18a, as shown in FIG. 2A. Then, if the inner leads are placed under a pressure, for example, when they are transported, burrs or flashing 18b extending in the transverse direction may be generated, as shown in FIG. 2B.
In the prior art, since an insulation film having substantially the same dimensions as the metal planes is used to adhered these planes to each other, the position thereof may become misaligned by about 75 .mu.m at maximum. Therefore, if the above-mentioned burrs or flashes are generated and if the positions of the metal planes are not aligned with respect to the insulating film, an electrical short circuit may occur due to a mechanical contact between the metal layers.
Further, to improve the electrical characteristic of the lead frame, a matching of the characteristic impedance has recently been required, and to effect this matching of the characteristic impedance, the thickness of the insulation film shall be reduced. In this case, an electrical short circuit still may easily occur between the metal layers, due to burrs or flashing produced by stamping.
Also, to improve the wire-bonding characteristic, the wire-bonding areas of the inner leads and the metal planes are frequently coated with a film of silver. This solution, however, has a problem in that the electrical insulation characteristic between the metal layers becomes lower due to a silver-migration, and if the thickness of the insulation film is reduced so that the distance between the metal layers becomes shorter, an electrical short circuit still may easily occur between the metal layers, due to this silver-migration.